The present invention relates in general to a method for the formation of orientation film of a liquid crystal display (hereinafter "LCD"), and more particularly to a method for forming an orientation film of photopolymer in an LCD, capable of providing a pretilt angle to the orientation film as well as adjusting it.
In a typical electrically switched LCD, a liquid crystal material is sealed between two glass or plastic plates that are provided with transparent conducting electrodes.
Hereinafter, a conventional LCD structure and method for fabricating the same will be discussed along with the problems generated therein with respect to the pretilt angle for better understanding of the background of the invention, referring to FIG. 1. There is a structure for a conventional LCD. As shown in FIG. 1, the inner surfaces of transparent glass substrates 1 and 1A are coated with transparent conducting electrodes 2 and 2A and are then covered with orientation films 3 and 3A, respectively. The inner surfaces are adhered to each other with a sealing material (not shown) so as to form a space, into which a liquid crystal material 4 is injected, the other surfaces of the glass substrates 1 and 1A being provided with polarizers 5 and 5A.
In such an LCD, it is required to align the liquid crystal injected between the two glass substrates in a uniform direction for a constant brightness and a high contrast ratio.
Several techniques for aligning the liquid crystal in a uniform direction are known. For example, there has been proposed such a method that an orientation film is deposited over a substrate which is subsequently subjected to the mechanical treatment of rubbing to form microgrooves, so that liquid crystal molecules may be aligned regularly on the entire surface of the orientation film therethrough. In this technique, a polyimide-type or polyamide-type polymeric material is widely used as a material for the orientation film. However, this technique employing the mechanical rubbing is disadvantageous in that the produced micro grooves inherently have defects, which cause random phase distortion and light scattering phenomena, thereby adversely affecting the operational characteristics of the display. In addition, static electricity generated during the mechanical rubbing of the polymer surface is believed to cause defects in active matrix displays. Furthermore, it is practically impossible to locally orient selected regions of the surface with the orientation of each different region.
In order to overcome the aforementioned disadvantages, a prepolymer, photosensitive material, has been proposed, which is polymerized by light to form the orientation film. The prepolymer includes polyvinyl-4-methoxycinnamic acid (hereinafter "PVCN-M") which is prepared by the reaction of polyvinyl alcohol (hereinafter "PVA") with 4-methoxy cinnamic acid. According to this technique, irradiation of linearly polarized ultraviolet (hereinafter "UV")light makes PVCN-M crosslinked, so that an oriented film of net structure is produced. This photopolymeric orientation film allows the liquid crystal molecules to be oriented in a preferred direction of planar structure. However, there exist problems in the orientation film formed by the photopolymerization of PVCN-M. For example, since the orientation film of PVCN-M has a bad thermostability, as ambient temperature becomes more than approximately 50.degree. C., distortion emerges therein. In addition, the orientation film provides zero pretilt angle. As a result of zero pretilt, the electro-optical characteristics of the LCD based on the photosensitive material are inferior to those based on the rubbed polymer. For example, higher driving voltages and worse contrast are shown.